Commodore Plus 4 - Page 251

The 6803 Microprocessor 239
The 6502 Microprocessor
The actual microprocessor chip used in the Plus/4 is a 7501. However, for
programming purposes, this chip is the same as a 6502. In this section, the
operation of the microprocessor is reviewed. Programmers familiar with any
machine language should be able to adapt to the 6502 after studying this section.
The examples in this section are designed to show the usage of the 6502
instruction set and can be entered (and saved on cassette or disk) using the built-in
machine-language monitor (see previous section).
Registers
The 6502 is equipped with six internal registers. These registers are accessed with
special instructions, which are not the same as instructions that access memory
locations. Three of these registers are user registers. It is usually faster to access
the registers than to access memory, so using them efficiently can speed up a
machine-language program.
The Program Counter The program counter (PC) is a 16-bit register that stores
the address of the next instruction to be executed. When an instruction is
executed, the PC is incremented so as to point to the following instruction. When
a branch or jump instruction is executed, the PC is changed accordingly.
The Status Register The status register (SR) is an 8-bit register containing the
status flags for the microprocessor. Only 7 bits are actually used, and they are
assigned as follows:
Bit Label Name Usage
7 N Negative This bit is set to 1 when the last result had the
high (sign) bit set to 1 and cleared to 0 when
the last result had the high bit cleared to 0.
6 V Overflow This bit is set to 1 when the last operation
resulted in a two's complement arithmetic
overflow. That is, two numbers with the
same sign were added, resulting in a number
of the opposite sign; a positive number was
subtracted from a negative number yielding
a positive number; or a negative number
was subtracted from a positive number
yielding a negative number. If no overflow
occurred, it is cleared to 0.